A magnetic random access memory and other such magnetic memories are memories each configured to record data as a direction of magnetization of a magnetic film. The magnetic film configured to record data as the direction of magnetization is hereinafter sometimes called “recording layer”. The magnetic memory has desired features as a memory, such as low-voltage and high-speed operation, non-volatility, and high rewrite endurance, and hence is being developed vigorously.
In early magnetic memories, data has been written by allowing an electric current to flow through wiring provided in the vicinity of the recording layer to generate a current-induced magnetic field (Oersted field), and switching the magnetization to a desired direction with the current-induced magnetic field (for example, see Patent Documents 1 and 2). However, the method of switching the magnetization with the current-induced magnetic field has a problem in that a write current is increased when a memory cell size is reduced.
In recent years, as a method of more efficient writing into the magnetic memory, there have been proposed methods of writing using spin carried by electrons. One of those methods of writing is a method of switching magnetization by allowing a spin-polarized current to flow in an in-plane direction of the recording layer to move a magnetic domain wall. In a magnetic memory using a perpendicular magnetization film as the recording layer, the method involving moving the magnetic domain wall with the spin-polarized current is considered to be especially useful. In recent years, various devices utilizing electron spin have been proposed, and such technology is sometimes called spintronics.
One of achievements in recent years in spintronics is that there has been found, as a method of achieving switching of magnetization of the perpendicular magnetic film, a phenomenon in which the magnetization of the perpendicular magnetic film is switched by applying a magnetic field to the perpendicular magnetic film in the in-plane direction, and allowing an electric current to flow through a heavy metal film (for example, Pt film or Ta film), which is joined to the perpendicular magnetic film, in a direction that is parallel to the magnetic field (for example, see Non Patent Documents 1 and 2). It is considered that this phenomenon in which the magnetization is switched is caused by an effect of a Rashba field, spin injection by the spin Hall effect, or both. This switching of magnetization is hereinafter sometimes called “in-plane current-induced perpendicular switching of magnetization”.
FIG. 1 is a conceptual diagram for illustrating the in-plane current-induced perpendicular switching of magnetization. Here, according to Non Patent Document 3, the phenomenon of the in-plane current-induced perpendicular switching of magnetization will be described as being caused by the spin injection by the spin Hall effect. In FIG. 1, the structure in which an adjacent layer 101 is joined to a perpendicular magnetic film 102 is illustrated. The perpendicular magnetic film 102 has perpendicular magnetization (magnetization in thickness direction), and the perpendicular magnetization is denoted by a reference symbol 102a. As the adjacent layer 101, a heavy metal film, for example, a Pt film or a Ta film, which strongly exhibits the spin Hall effect, is used. In FIG. 1, there is defined an xyz orthogonal coordinate system in which an x-axis and a y-axis are defined in an in-plane direction of the adjacent layer 101 and the perpendicular magnetic film 102, and in which a z-axis is defined in a thickness direction of the adjacent layer 101 and the perpendicular magnetic film 102, and in the following description, this xyz orthogonal coordinate system is sometimes used to indicate directions.
In the structure of FIG. 1, when an electric current Iy is allowed to flow through the adjacent layer 101 in parallel to the y-axis direction while applying a magnetic field Hy to the perpendicular magnetic film 102 in parallel to the y-axis direction, switching of magnetization occurs. A direction in which the magnetization of the perpendicular magnetic film 102 is switched depends on directions of the magnetic field Hy, the electric current Iy, and a spin current generated by the spin Hall effect. The direction of the spin current depends on the direction of the electric current Iy and a sign of a spin Hall angle of the adjacent layer 101. For example, in a case where the magnetization of the perpendicular magnetic film 102 is initially directed to a +z direction, and a Pt film is used as the adjacent layer 101, when the electric current Iy (>0) is allowed to flow through the adjacent layer 101 in a +y direction while applying the magnetic field Hy (>0) to the perpendicular magnetic film 102 in the +y direction, the magnetization of the perpendicular magnetic film 102 is switched from the +z direction to a −z direction. Such switching of magnetization cannot be explained as the switching of magnetization by the current-induced magnetic field, which is caused by allowing the electric current to flow through the adjacent layer 101, or the switching of magnetization by the injection of the spin-polarized current.
According to Non Patent Document 3, the above-mentioned in-plane current-induced perpendicular switching of magnetization is theoretically described as follows. A behavior of magnetization of a magnetic film is expected to follow the following Landau-Lifshitz-Gilbert equation (LLG equation):
                              dm          dt                =                              γτ            tot                    +                      α            ⁡                          (                              m                ×                                  dm                  dt                                            )                                                          (        1        )            
In the equation, m represents a unit magnetization vector, and components of the unit magnetization vector m in the xyz orthogonal coordinate system are m=(mx, my, mz). Moreover, γ represents a gyromagnetic ratio, a represents a damping constant, and τtot represents total torque that acts on the magnetization.
The total torque τtot is obtained by the following equation (2):τtot=τst+τext+τan  (2)In other words, the total torque τtot is expressed as a sum of Slonczewski-like torque τst including a contribution from the spin Hall effect induced by the electric current, torque τext induced by an external magnetic field, and torque τan caused by magnetic anisotropy.
Here, terms of the above-mentioned equation (2) are expressed as the following equations (3), (4), and (5), respectively:τst=τst0(m×x×m)  (3)τext=−m×Bext  (4)τan=−Ban0mz(m×z)  (5)In the equations, τ0st represents the Slonczewski-like torque, and is proportional to the write current. Bext represents a vector of the external magnetic field, and components of the vector in the xyz orthogonal coordinate system are Bext=(Bx, By, Bz). Moreover, x represents a unit vector in an x direction, y represents a unit vector in a y direction, and z represents a unit vector in a z direction. B0an represents an anisotropic magnetic field. In Non Patent Document 3, it is reported that the equation (1) well explains the in-plane current-induced perpendicular switching of magnetization. As the Slonczewski-like torque, in addition to the contribution by the spin Hall effect, according to Non Patent Document 1, a contribution from a Rashba field, which is generated by an electric current flowing through a recording layer, is suggested.
The inventors of this invention consider the use of the above-mentioned in-plane current-induced perpendicular switching of magnetization for writing data into a recording layer of a memory cell of a magnetic memory (that is, switching of magnetization). Using the in-plane current-induced perpendicular switching of magnetization to write data may improve data writing properties.
One of problems in writing data using the in-plane current-induced perpendicular switching of magnetization is that an intermediate state (state in which whether “0” or “1” is indicated is indefinite) may exist in the recording layer. For example, in Non Patent Document 3, it is disclosed that an intermediate state may exist when the magnetization is switched using the in-plane current-induced perpendicular switching of magnetization. The existence of the intermediate state is undesirable in writing data.
Non Patent Document 4 relates to a writing method using the in-plane current-induced perpendicular switching of magnetization, and it is demonstrated by simulation that miswriting can be reduced by increasing a damping constant.
Moreover, in Patent Document 3, there is disclosed a technology of controlling a magnetization state of a ferromagnetic material by applying a current pulse while externally applying a weak magnetic field to the ferromagnetic material.